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    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
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<body>
    <script>
        var a = 123;
        a.len = 10;//new Number(a) 会被delete 因为没有地方保存
        console.log(a.len)//undefined
        var x = y = 4;
        console.log(x, y)


        function foo(d, b, c) {
            d = 10
            return arguments[2]
        }
        foo(1, 2, 3)
        // console.log(d) //not defined
    </script>
</body>

</html>